Leaky Gut (Increased Intestinal Permeability): A Comprehensive Reference
1. Definition and Conceptual Status
"Leaky gut" refers to the dysfunction of the intestinal barrier and often leads to the generation of leaky gut syndrome (LGS) under chronic states. The pathological manifestation of leaky gut is increased intestinal permeability, induced by various causative factors. More precisely, the term "leaky gut" is used to indicate abnormal translocation of large-size molecules from lumen to villi, or excessive absorption of such molecules from the lumen into systemic circulation, which in turn may induce various organ disorders.
Disruption of the barrier is referred to as "leaky gut" or leaky gut wall syndrome and is characterized by the release of bacterial metabolites and endotoxins, such as lipopolysaccharide (LPS), into the circulation. The term is widely used in nutrition and natural health contexts; however, many such factors have been proposed to cause leaky gut syndrome, with stress and gluten consumption commonly highlighted. Despite the resulting plethora of lifestyle and diet recommendations that abound on the internet, a closer examination of these claims alongside the existing literature raises doubts about their validity.
The scientific and clinical literature consistently distinguishes between the measurable phenomenon of increased intestinal permeability — a physiologically and experimentally defined state — and the broader popular concept of "leaky gut syndrome" as a standalone diagnosis. The objectives of authoritative reviews on "leaky gut" for clinicians include discussing the components of the intestinal barrier, the diverse measurements of intestinal permeability, their perturbation in non-inflammatory "stressed states," and the impact of treatment with dietary factors. Information on "healthy" or "leaky" gut in the public domain requires confirmation before endorsing dietary exclusions, replacement with non-irritating foods, or use of supplements to repair the damage.
2. Anatomy and Physiology of the Intestinal Barrier
2.1 Structural Layers of the Barrier
The intestinal barrier includes the surface mucus, the epithelial layer, and immune defences. The intestinal barrier is a dynamic system influenced by the composition of the intestinal microbiome and the activity of intercellular connections, regulated by hormones, dietary components, inflammatory mediators, and the enteric nervous system (ENS).
The gut lumen is where the degradation of bacteria and antigens by bile, gastric acid, and pancreatic juice occurs. Commensal bacteria inhibit the colonization of pathogens through production of defense antimicrobial substances. The unstirred water layer, glycocalyx, and thick/thin mucus layer work to prevent bacterial adhesion through immunoglobulin A (IgA) secretion. Within the epithelium, tight junctions (TJs) connect the epithelial cells to transport luminal contents.
2.2 Tight Junctions: The Molecular Gate
Epithelial tight junctions (TJs) maintain the intestinal barrier while regulating permeability of ions, nutrients, and water. The TJ is a multi-protein complex that forms a selectively permeable seal between adjacent epithelial cells and demarcates the boundary between apical and basolateral membrane domains. TJ-associated proteins, including occludin, zonula occludens-1 (ZO-1), claudin-1/4, and junctional adhesion molecules, are vital to barrier integrity.
Long thought to be a static, non-regulated barrier to the passage of luminal material, the tight junction is now recognised to be a dynamic, constantly changing structure with a functional state that is carefully regulated. Luminal organisms can modulate the state of the tight junction through multiple mechanisms; while opening tight junctions may be of benefit for the microflora, it may be deleterious to the host.
Molecules can cross the tight junctions via two distinct size-selective and charge-selective paracellular pathways: the pore pathway and the leak pathway. These can be distinguished by their selectivities and differential regulation by immune cells.
2.3 Routes of Increased Permeability
Epithelial permeability results from increased paracellular transport, apoptosis, or transcellular permeability. The major determinant of the rate of intestinal permeability is the opening or closure of the tight junctions between enterocytes in the paracellular space.
2.4 The Zonulin Pathway
In many scenarios where permeability is increased, a common pathophysiological event is the upregulation of zonulin secretion from a lamina propria source into the lumen, with inappropriate activation of this pathway. The end result is increased paracellular permeability. Zonulin is a modulator of tight junction function that was identified through research on bacterial toxins and has become a biomarker studied in the context of intestinal permeability.
2.5 Measurement Methods
Barrier function can be tested in vivo using orally administered probe molecules, or in vitro using mucosal biopsies from humans, exposing the colonic mucosa from rats or mice, or cell layers to extracts of colonic mucosa or stool from human patients. In humans, lactulose/mannitol or lactulose/rhamnose tests have been used to assess intestinal permeability by measuring the urinary excretion of unabsorbed lactulose and absorbed mannitol or rhamnose. The lactulose/mannitol or lactulose/rhamnose ratio is increased in patients with multiple sclerosis, rheumatoid arthritis, type 1 diabetes, or celiac disease. Consumer blood or stool kits marketed for "leaky gut," especially zonulin-based tests, are not considered well validated for diagnosing it.
3. Body Systems Involved
Disruption of the intestinal TJ barrier, followed by permeation of luminal noxious molecules, induces a perturbation of the mucosal immune system and inflammation, which can act as a trigger for the development of intestinal and systemic diseases.
Intestinal hyperpermeability can allow the entry of harmful agents through the junctions of the intestinal epithelium, which pass into the bloodstream and affect various organs and systems. A damaged intestinal barrier can facilitate the development of local diseases such as irritable bowel disease, inflammatory bowel disease, or celiac disease, but also the development of systemic inflammatory diseases such as rheumatoid arthritis, ankylosing spondylitis, hepatitis, and lupus erythematosus, neurodegenerative or psychiatric conditions, or metabolic diseases such as diabetes or obesity. However, it must be emphasized that the causal links between a leaky gut barrier and the onset of certain diseases often remain unclear.
The literature identifies several axes through which gut barrier dysfunction may interact with systemic health:
- Gut–liver axis: There is growing evidence suggesting that dysbiosis in gut microbiota and metabolites disrupt the integrity of the intestinal barrier and significantly impact the level of inflammation in various tissues, including the liver and adipose tissues.
- Gut–brain axis: Data suggest that an imbalance of the intestinal microbiota, known as dysbiosis, may contribute to a variety of somatic diseases such as obesity, type 2 diabetes, inflammatory bowel diseases, and allergy. Recent studies suggest that gut bacteria also influence brain functions and behavior and may therefore play a role in the development of psychiatric disorders.
- Immune system: Clinical reports have suggested that increased intestinal permeability (LGS) contributes to autoimmune diseases such as type 1 diabetes, multiple sclerosis, rheumatoid arthritis, and celiac disease. The gut commensal microbiota plays a critical role in regulating host immunity; abnormalities of the microbial community, known as dysbiosis, are observed in patients with autoimmune diseases. However, the pathological links among intestinal dysbiosis, LGS, and autoimmune diseases have not been fully elucidated.
4. Contributing and Associated Factors
4.1 Gut Microbiota Dysbiosis
The intestinal microbiota is essential to maintain the intestinal epithelium's integrity and homeostasis. A qualitative and quantitative imbalance in the composition of the intestinal microbiota — dysbiosis — contributes to intestinal barrier dysfunction and leaky gut syndrome. Recent studies have revealed that gut microbiota affects intestinal and systemic health conditions via their metabolites, especially short-chain fatty acids and lipopolysaccharides, which can trigger leaky gut.
4.2 Dietary Factors
Diet can be considered one of the main modulating factors of gut microbiota, impacting its composition and functionality. Excessive consumption of simple carbohydrates, saturated fats, and processed foods appears to be directly linked to dysbiosis, which can lead to intestinal hyperpermeability and leaky gut syndrome.
Recent research has shown that consuming highly refined carbohydrates can increase the production of proinflammatory cytokines and decrease the gene expression of TJ proteins. Several dietary components are detrimental to the barrier, including ethanol, fat, sugars, gliadin, food additives, emulsifiers, and microbial transglutaminase.
Regarding food additives specifically, food additives have been related to permeable bowel syndrome. A recent review describes the ability of additives to increase intestinal permeability by interfering with TJs, promoting the passage of antigens immunogenic to the organism.
4.3 Alcohol
This condition is mainly caused by bacterial infections, oxidative stress, high-fat diet, exposure to alcohol or chronic allergens, and dysbiosis. Alcohol dependence research has shown that alcohol consumption was associated with gut leakiness, gut-microbiota alterations, and affective symptoms. After three weeks of abstinence, affective symptoms recovered completely in the subset of subjects that did not present with gut dysfunction.
4.4 Pharmaceutical Agents
The central mechanism of nonsteroidal anti-inflammatory drugs (NSAIDs) suggests induced intestinal hyperpermeability and causes small bowel disease. NSAIDs damage enterocytes through biochemical processes. Oral antibiotics can cause acute or long-term effects due to eliminating antiproteolytic bacteria and increasing proteolytic activities in the colon. In general, antibiotics change gut microbiota, implicate dysbiosis, impair gut barrier, and cause intestinal inflammation.
4.5 Psychological and Physiological Stress
Tight junction proteins play a role in the control of intestinal permeability and can be disrupted by stress through signal pathways triggered by the ligation of receptors with stress hormones. An impaired intestinal barrier function can be detrimental to the host as it may allow the translocation of luminal antigens and toxins into the subepithelial tissue and bloodstream, which may cause local and systemic immune responses and lead to the development of pathologies. In vitro and animal studies strongly suggest that psychosocial stress is one of the factors that can increase intestinal permeability via mast-cell dependent mechanisms. Direct human evidence for psychosocial stress acutely increasing intestinal permeability remains limited.
4.6 Intense Physical Exercise
The effects of physical exercise depend on intensity. Endurance athletes have a high incidence of gastrointestinal disorders, and the "leaky" gut is one of the most common disorders. It is characterized by dysfunction of the intestinal epithelial barrier and its excessive permeability. In healthy young adult male cyclists, just one hour of physical activity at 70% maximum workload capacity produced splanchnic hypoperfusion, which can cause decreased gastrointestinal circulation, increased intestinal permeability, and damage to the small intestine. Another study showed humans exercising at 70% VO2max presented a 60–70% reduction in splanchnic blood flow, and exercise-induced ischemia caused increased intestinal permeability when blood flow was reduced by 50%. By contrast, specific diets and regular low- and moderate-intensity exercises are proposed as effective non-pharmacological approaches to maintain the integrity of the intestinal wall and its efficient operation.
4.7 Enteric Pathogens
Enteric pathogens can bind to the cell surface and induce changes in the expression of tight junction proteins. In addition, the production of toxins by pathogens can promote cellular damage through disruption of intracellular protein interactions, leading to increased cellular permeability and ultimately triggering cell death. While some pathogens primarily use one mechanism to alter host physiology, others, including Salmonella and Escherichia coli, are capable of altering the cellular functions of the intestinal epithelium through multiple mechanisms.
4.8 Hyperglycemia and Metabolic Disease
Several studies show that hyperglycemia, a key feature of diabetes, induces intestinal barrier dysfunction. Prolonged exposure to glucose at high levels increases migration capacity of human colonic cell line Caco-2, resulting in layers appearing less organized than under physiological conditions.
5. Nutrients, Herbs, and Natural Ingredients in Relation to Leaky Gut
The following section separates traditional uses from scientific evidence and clearly characterizes the strength of the available evidence for each substance.
5.1 L-Glutamine
Traditional/Background Use: Glutamine has long been used clinically in parenteral and enteral nutrition protocols for critically ill and surgical patients, based on the understanding that it is a primary energy substrate for rapidly proliferating enterocytes. Its use in oral supplementation for gut health in general wellness contexts developed later, drawing on these clinical observations.
Scientific Evidence: A randomized, controlled trial comparing glutamine supplement with standard formula for 10 days in malnourished children showed that the L/M ratio decreased significantly in the glutamine-treated group compared with the same standard formula plus glycine, demonstrating an improvement in gut permeability. Another randomized, controlled trial showed that glutamine improved intestinal permeability, assessed by the L/M ratio test, and morphology in patients with Crohn's disease; however, the control group receiving whey protein (to match net protein intake) had similar results.
A 2024 systematic review and meta-analysis (10 RCTs, 352 participants) provided important context for the overall evidence base: Overall, glutamine supplementation did not significantly affect intestinal permeability (WMD: −0.00, 95% CI −0.04, 0.03). Subgroup analysis showed a significant reduction in intestinal permeability with doses over 30 g/day. Evidence strength: Mixed in healthy or general populations; modest and context-dependent positive signal at high doses or in specific clinical scenarios (critical illness, post-infectious IBS, malnourished children). Most evidence comes from small trials with heterogeneous populations.
5.2 Zinc
Traditional/Background Use: Zinc has been used in traditional medicine systems across multiple cultures as a wound-healing and anti-infective mineral. Its application to gut health is an extension of modern nutritional medicine, not a traditional herbal practice.
Scientific Evidence: A study of ZIP14 knockout mice showed increased intestinal permeability by serum FITC-dextran levels and endotoxins, reduced expression of phosphorylated occludin and claudin-1, and increased claudin-2. These findings suggest a role for zinc in maintaining intestinal barrier function. In vitro studies show that vitamin A and vitamin D modify the expression of TJ molecules. Intestinal epithelial cell lines treated with vitamin A or D have shown increased transepithelial electrical resistance (TEER), with upregulation of ZO-1, occludin, and several claudins. One patient-based study focused on whether orally administered zinc gluconate (26 mg twice daily) can induce the remodeling of gastrointestinal barrier function and reduce passive leak across the human intestinal mucosal barrier in situ. Evidence strength: Animal and cell culture data are suggestive of a barrier-protective role. Human clinical trial data specific to gut permeability are limited. Zinc deficiency is associated with impaired barrier function in observational and cross-sectional studies.
5.3 Vitamin D
Traditional/Background Use: Vitamin D is not a traditional herbal remedy but a fat-soluble vitamin whose role in gut health has become a focus of modern nutritional research alongside its established functions in calcium metabolism and immune regulation.
Scientific Evidence: The potential association between vitamin D deficiency and intestinal barrier function has been tested in experimental animals. Short-term treatment with 2,000 IU/day of vitamin D or placebo for 3 months in patients with Crohn's disease has shown that vitamin D may improve gastroduodenal permeability (sucrose excretion) relative to the deterioration observed on placebo; however, there was no significant improvement in all measures studied. Epithelial cells and several types of immune cells in the gastrointestinal tract express vitamin D and vitamin A receptors. Evidence strength: Preclinical and mechanistic evidence is substantial. Clinical trial data in humans are preliminary, with small sample sizes. A 2023 Cochrane review on vitamin D for inflammatory bowel disease has been published but broader, permeability-specific RCT data remain limited.
5.4 Short-Chain Fatty Acids (SCFAs) and Butyrate
Traditional/Background Use: SCFAs are not taken as traditional herbal preparations; however, the dietary practice of consuming fermented foods, wholegrains, and legumes — common across many traditional food cultures — increases colonic SCFA production via microbial fermentation.
Scientific Evidence: Butyrate, a short-chain fatty acid derived from microbial fermentation of dietary fibers in the colon, has been described as an intestinal barrier-strengthening agent, although mainly based on in vitro and animal models. One study aimed to investigate butyrate's ability to prevent intestinal hyperpermeability in human colonic tissues. Butyrate can improve paracellular permeability by modulating hypoxia-inducible factor-1 and epithelial tight junction CLDN1. Butyrate is known to be a histone deacetylase inhibitor with reported anti-inflammatory, oxidative stress-reducing, and intestinal barrier-strengthening effects. Several studies using cell culture models and animal models have shown that butyrate can strengthen barrier function and decrease intestinal permeability. Dietary fiber is digested by enzymes and microorganisms fermenting short-chain fatty acids such as butyrate and propionate, which are key factors in the protection of the intestine. Evidence strength: Mechanistic and animal data are strong; direct human clinical trial evidence for oral butyrate supplementation improving intestinal permeability biomarkers is emerging but not yet definitive. Dietary fiber as a precursor to colonic butyrate production has more established indirect support.
5.5 Polyphenols (Quercetin, Curcumin, Resveratrol, EGCG, Berberine)
Traditional Use: Curcumin (from turmeric, Curcuma longa) has been used for centuries in Ayurvedic and Traditional Chinese Medicine (TCM) for inflammatory gastrointestinal complaints. Berberine, found in plants such as Berberis vulgaris (barberry) and Coptis chinensis, has been used for thousands of years in TCM and Ayurveda for diarrhea and intestinal infections. Quercetin is abundant in onions, apples, and capers and is consumed as part of traditional diets globally. Resveratrol is found in grapes and has been associated with traditional wine-drinking cultures in the Mediterranean. Green tea (Camellia sinensis), the source of EGCG (epigallocatechin gallate), has been consumed for millennia in East Asian traditions for its digestive and health-promoting properties.
Scientific Evidence: Polyphenols have been shown to enhance tight junction integrity, increase mucus secretion, and decrease intestinal barrier permeability, thereby generally improving the intestinal defense mechanism. In addition to the involvement of polyphenols in multiple inflammatory signaling pathways, they also exert beneficial effects by acting on the intestinal epithelium. Recent studies indicate that a polyphenol-rich diet lowers the risk of intestinal barrier dysfunctions. Polyphenols such as quercetin, EGCG, catechin, epicatechin, berberine, resveratrol, and curcumin have been studied intensely to provide health benefits in leaky gut-related diseases. In vivo as well as in vitro experiments have shown that curcumin and resveratrol have barrier-protective properties during a Campylobacter jejuni infection. Evidence strength: Largely preclinical (cell culture and animal models). Human clinical trials examining these polyphenols specifically on intestinal permeability biomarkers are sparse. Most human evidence pertains to inflammatory biomarkers in IBD or metabolic conditions rather than direct permeability measures. Overall evidence is preliminary.
5.6 Probiotics
Traditional Use: Fermented foods containing live microorganisms — such as yogurt, kefir, kimchi, sauerkraut, miso, and tempeh — have been integral to food traditions across many cultures for centuries, long before the concept of probiotics was established. Their consumption for digestive health is documented in folk and traditional medicine across European, Asian, and Middle Eastern traditions.
Scientific Evidence: A systematic review and meta-analysis of RCTs found that probiotics potently decreased the level of endotoxin (SMD, −3.20, 95% CI, −5.41 to −0.98, P = 0.005) in four studies. These findings suggested that probiotics could improve intestinal barrier function to some extent, but more high-quality RCTs are needed to achieve a solid conclusion. Further in-depth research is required to target the precise dose, intervention duration, and strains of probiotics to provide valuable instructions for clinical practice.
A separate review of probiotic use in overweight and obese individuals found: eleven of 12 animal studies reported a positive effect of probiotic supplementation in reducing intestinal permeability. However, results from clinical trials were inconsistent, with half reporting reductions in serum LPS and half reporting no differences after probiotic supplementation. Bifidobacterium, Lactobacillus, and Akkermansia emerged as the most common genera in probiotic formulations among the animal and clinical studies that yielded positive results, suggesting that specific bacteria may be more effective at reducing intestinal permeability and improving gut barrier function.
Probiotics exert direct and indirect antagonism of pathogens, and there are documented effects of diverse probiotic species, especially combination agents, on barrier function in vitro, in vivo in animal studies, and in human randomized controlled trials conducted in response to stress or disease. Clinical observations of benefits with combination probiotics in inflammatory diseases have simultaneously not appraised effects on intestinal permeability. In summary, probiotics and synbiotics enhance intestinal barrier function in response to stressor or disease states. Evidence strength: Mixed in healthy adults; positive signals from animal studies and some RCTs in disease populations. Evidence for specific strains, doses, and durations is insufficient to make generalised claims.
5.7 Dietary Fiber and Prebiotics
Traditional Use: High-fiber diets based on whole grains, legumes, vegetables, and fruits are the nutritional norm in many traditional food cultures worldwide and predate any modern understanding of intestinal permeability.
Scientific Evidence: A systematic review of dietary interventions in healthy populations found that chicory inulin and probiotics reduced intestinal barrier permeability in adults with a moderate GRADE level of evidence. The opposite result was obtained with fructose, which increased intestinal barrier permeability in adults, with a very low GRADE level of evidence. Prebiotic dietary fibers (e.g., inulin, starch, and fructooligosaccharides), probiotics (live microorganisms), and synbiotics (probiotic-prebiotic combinations) represent promising therapeutic nutrients that modulate intestinal permeability through tight junction protein regulation. Studies indicate that probiotic administration can shift the microbial balance toward beneficial species (e.g., bifidobacteria and lactobacillus), potentially reducing zonulin-activating compounds. Evidence strength: Moderate for inulin-type fructans in reducing permeability; the overall evidence for diverse fiber types as direct modulators of intestinal permeability in healthy humans requires further research with lower risk of bias.
5.8 Vitamin A
Traditional/Background Use: Dietary intake of vitamin A (from animal sources as retinol, and from plant sources as beta-carotene) has been a cornerstone of nutritional adequacy in traditional diets globally.
Scientific Evidence: In a cross-sectional study of Brazilian children, higher intestinal permeability (L/M ratio) was documented in children with vitamin A deficiency. A randomized, controlled trial of four months' duration conducted in children compared the effects of vitamin A treatment with placebo and documented no differences in L/M ratio, although the vitamin A-treated group had significantly lower urinary lactulose and mannitol excretion. This was associated with lower prevalence of parasitic and Giardia infections, although the relationship between reduced infections and changes in barrier function is unclear. Evidence strength: Deficiency of vitamin A is associated with barrier dysfunction in observational data; RCT evidence for supplementation improving permeability specifically is weak and limited to specific vulnerable populations.
5.9 Bovine Colostrum
Traditional/Background Use: Colostrum — the first milk produced by mammals after birth — has been consumed in various traditional cultures, and bovine colostrum has been used in Ayurvedic medicine. Modern supplementation use focuses on its bioactive constituents, including immunoglobulins, lactoferrin, and growth factors.
Scientific Evidence: Bovine colostrum (BC) contains a myriad of bioactive molecules that are renowned for possessing unique medicinal benefits in children and adults, and BC supplements are considered safe and cost-effective options to manage/prevent the incidence of upper respiratory tract infections and gut-related problems in athletes. A systematic review using PRISMA guidance (9 eligible studies) specifically examined bovine colostrum supplementation's influence on gut permeability, particularly in athletes. Prolonged exercise, especially in excessively hot environment conditions, has shown to cause increased gut permeability. Evidence strength: Preliminary; the number of qualifying trials is small, populations are specific (athletes), and the overall evidence base does not yet support generalised claims about colostrum for gut permeability in the broader population.
5.10 Tryptophan and Other Amino Acids
Dietary factors that can reverse stressor-induced barrier disruption include supplements such as zinc or glutamine, which are among the substances that enhance the barrier. Other dietary factors in the diet that improve the barrier are vitamins A and D, tryptophan, cysteine, and fiber. Research on tryptophan's role is largely mechanistic and preclinical; its metabolism via the aryl hydrocarbon receptor and serotonin pathways intersects with intestinal immune function and barrier regulation, but targeted human RCT evidence for improving gut permeability is not yet established.
6. Dietary Patterns and Lifestyle Factors
6.1 The Western Diet as a Risk Pattern
Excessive consumption of simple carbohydrates, saturated fats, and processed foods appears to be directly linked to dysbiosis, which can lead to intestinal hyperpermeability and leaky gut syndrome. Several research studies have demonstrated that high-fat diets (HFDs) elevate the levels of endotoxin in the body's circulation, leading to a decrease in the expression of tight junction proteins such as zonula occludens (ZO)-1, occludin, and claudin.
6.2 The Mediterranean Diet as a Protective Pattern
Characterized by a high intake of plant-based foods, monounsaturated fats, and polyphenols, primarily from extra virgin olive oil, the Mediterranean diet (MD) fosters the growth of beneficial gut bacteria such as Bifidobacterium, Faecalibacterium prausnitzii, and Roseburia, which produce short-chain fatty acids that enhance gut barrier integrity, reduce inflammation, and improve metabolic homeostasis. Clinical and preclinical studies have proved that the MD is associated with increased microbial diversity, reduced pro-inflammatory bacteria, and improved markers of insulin sensitivity, lipid metabolism, and cognitive function. Direct RCT evidence linking Mediterranean diet adherence to reduced intestinal permeability measurements specifically is limited.
6.3 Specific Dietary Components to Avoid
Dietary factors such as ethanol, bile acids, and emulsifiers have been shown to increase intestinal permeability via a number of different mechanisms in animal and/or human studies. Ethanol, fructose, and dietary emulsifiers increase permeability. It should be emphasised that the aforementioned dietary factors have not been studied specifically for the treatment of a pathologic disorder associated with leaky gut syndrome, and thus a direct link between diet and treatment of leaky gut syndrome has not been established.
6.4 Gluten and Gliadin
Gliadin, a component of wheat gluten, has been identified in research contexts as a stimulus for zonulin release and increased permeability. Wheat proteins are responsible for celiac disease and other gluten-/wheat-sensitive or intolerance conditions like IBD and IBS, and are associated with many other diseases due to chronic inflammation. The applicability of these findings to healthy individuals without celiac disease or non-celiac wheat sensitivity remains a matter of ongoing scientific debate.
6.5 Exercise: Intensity-Dependent Effects
The effects of physical exercise on gut permeability depend on their intensity. Endurance athletes have a high incidence of gastrointestinal disorders, and leaky gut is one of the most common. It is characterised by dysfunction of the intestinal epithelial barrier and its excessive permeability. Conversely, specific diets and regular low- and moderate-intensity exercises are proposed as effective non-pharmacological approaches to maintain the integrity of the intestinal wall and its efficient operation.
6.6 Psychological Stress and the Brain–Gut Axis
In vitro and animal studies strongly suggest that psychosocial stress is one of the factors that can increase intestinal permeability via mast-cell dependent mechanisms. Evidence has been collected on several lifestyle pillars that influence the gut microbiome, including dietary intakes, exercise, sleep, stress, and toxin exposures. Direct prospective human evidence for stress-reduction interventions improving measurable intestinal permeability remains limited and requires further investigation.
6.7 Akkermansia muciniphila as an Emerging Focus
Recent studies have shown that the proportion of Akkermansia muciniphila, a member of the Verrucomicrobia phylum, is reduced in individuals with obesity and diabetes, as well as in corresponding mouse models. Additionally, it has been found that the presence of A. muciniphila, regardless of its viability, is linked to improvements in the integrity of the intestinal mucosal barrier, an increase in goblet cells, and enhanced metabolic functions. This association is currently the subject of active preclinical and early clinical research.
7. Key Evidence Gaps and Interpretive Cautions
The etiology of leaky gut is still unknown; however, recent studies have uncovered exogenous factors that can modulate intestinal permeability. The field faces several structural challenges. Most mechanistic data originate from cell culture and animal models, which do not always translate to human physiology. Human clinical trials are frequently small, heterogeneous in population and methodology, and use different permeability measurement tools, making cross-study comparison difficult. Diet would seem a logical target for the treatment of intestinal permeability given the inherent direct interaction between food, the gut epithelial lining, and potential changes in the integrity of the intestinal epithelial barrier. Yet it should be emphasised that the aforementioned dietary factors have not been studied specifically for the treatment of a pathologic disorder associated with leaky gut syndrome, and thus a direct link between diet and treatment of leaky gut syndrome has not been established.
Increased intestinal permeability occurs in a wide range of disorders, including inflammatory bowel disease, coeliac disease and graft-versus-host disease, but the relative contributions of barrier dysfunction and immune responses are unclear. This makes it difficult to determine in many conditions whether increased permeability is a primary driver, a contributing factor, or a consequence of disease.
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